The two major objectives of this proposal are: 1) to evaluate the importance of marrow-dependent M cells in host defense against certain infectious agents, particularly those encountered in marrow transplant recipients treated for leukemia, and 2) to evaluate the mechanism of immunosuppression during graft-versus-host disease, a major complication of marrow transplantation. M cells will be depleted by injecting mice with SR89, a long-lived, bone-seeking isotope. Proliferation of injected organisms, e.g., Listeria monocytogenes, cytomegalovirus, will be quantitated during early stages of infection, since preliminary experiments indicated that M cells function at that time. A cell-free factor from T cells that suppresses antibody responses during graft-versus-host reactions will be tested for its effect on M cell function and resistance to infection by selected infectious agents.